THIS INVENTION relates to a method of making analogues of .beta."-alumina in which the Na ions in .beta."-alumina are at least partially replaced by other metal cations; to a method of making polycrystalline artifacts from such analogues; and to products produced by said method.
.beta."-alumina (ie sodium .beta."-alumina) is a sodium aluminate having an idealized chemical formula Na.sub.2 O.5Al.sub.2 O.sub.3 with a layered structure in which the sodium ions are present in discrete layers separated from one another by layers comprising aluminium ions and oxygen ions, forming a spinel-type structure. In the analogues of .beta."-alumina produced in accordance with the present invention, at least some of the Na ions in said layers thereof are replaced by other cations.
.beta."-alumina can be made by dispersing in aluminium oxide (Al.sub.2 O.sub.3) or a precursor thereof, sodium oxide (Na.sub.2 O or soda) in finely divided particulate form to provide an intimate starting mixture, and heating the mixture to a conversion temperature at which at least part of the mixture is converted to .beta."-alumina. Optionally, particularly when it is intended to form a polycrystalline artifact from the .beta."-alumina by sintering the mixture to a sintering temperature, above the conversion temperature at which the conversion to .beta."-alumina takes place and at which sintering temperature the .beta."-alumina is unstable and can revert to .beta.-alumina (whose idealized chemical formula is Na.sub.2 O.11Al.sub.2 O.sub.3 and which has a similar layered spinel-type structure in which layers of Na ions are separated by layers comprising Al and O ions but which has a higher resistivity than .beta."-alumina), a spinel stabilizer or spinel-forming oxide such as Li.sub.2 O or MgO (lithia or magnesia) can be admixed in the starting mixture, the spinel stabilizer acting to stabilize the .beta."-alumina at the sintering temperature and also thereafter, when the polycrystalline artifact is cooled.
When .beta."-alumina is formed by heating to the conversion temperature, and is then further heated in the absence of a spinel stabilizer or spinel-forming oxide, it passes through a transition temperature, above the conversion temperature, above which transition temperature the .beta."-alumina reverts to .beta.-alumina.
Thus, by a `spinel-forming oxide` or `spinel stabilizer`, examples of which are lithia and magnesia, is meant an oxide, which, when dispersed in .beta.-alumina, promotes the stabilization therein of any .beta."-alumina above the transition temperature. Without the presence of a spinel stabilizer any .beta."-alumina typically reverts to .beta.-alumina above the transition temperature, so that it is difficult or impossible, in the absence of a spinel stabilizer, to make a .beta."-alumina-containing sintered ceramic artifact by heating a .beta.-alumina-containing powder compact to a sintering temperature which is above the transition temperature.